1. Field of the Invention
The present invention relates to a driving control circuit and a liquid crystal display (LCD) comprising the driving control circuit. More particularly, the present invention relates to a driving circuit for modulating a common voltage and an LCD comprising the driving circuit.
2. Descriptions of the Related Art
FIG. 1 depicts the pixel circuit 1 of a prior art liquid crystal display (LCD). The pixel circuit 1 comprises a thin-film transistor (TFT) 11, a liquid crystal capacitor 12, and a storage capacitor 13. The liquid crystal capacitor 12 and the storage capacitor 13 have a common voltage terminal 121 and 131 respectively to receive a common voltage. The TFT 11 receives a control signal transmitted by a scan line (not shown) via the gate G thereof. When the control signal from the scan line turns on the TFT 11, the data on a data line (not shown) is written into the liquid crystal capacitor 12. Simultaneously, a data voltage is stored in the storage capacitor 13 so that a continued supply of the data voltage is maintained across the liquid crystal capacitor 12 after the TFT 11 is turned off.
Unfortunately, there is a parasitic capacitance 14 between the gate G and the drain D of the TFT 11. Consequently, when the control signal received at the gate G transitions from a positive level to a negative level or vice versa, the voltage difference will be coupled to the storage capacitor 13 and thus, alters the voltage across the storage capacitor 13. This reaction is known as the feed-through effect. Because the feed-through effect tends to cause variation of the voltage stored in the storage capacitor 13, i.e., variation of the data voltage originally written, the display quality of the LCD image may be poor.
As shown in both FIGS. 2A and 2B, FIG. 2A depicts a pixel circuit 2 of the prior art aimed to overcome the feed-through effect, while FIG. 2B depicts the timing diagram of the pixel unit 2 of FIG. 2A. The pixel circuit 2 comprises a TFT 21, a liquid crystal capacitor 22, and a storage capacitor 23; the connections among which are just the same as the counterparts in FIG. 1. In particular, the TFT 21 is coupled to the data line 24, while the liquid crystal capacitor 22 has a common voltage terminal 221 that receives the direct current (DC) common voltage. The storage capacitor 23 has a common voltage terminal 231 that receives an alternating current (AC) common voltage.
In FIG. 2B, G1 represents a scan signal transmitted to the TFT 21, G2 represents a scan signal transmitted to the TFT at the next stage (not shown), VCOM-AC represents a waveform of the VCOM signal supplied to the common voltage terminal 231, and Vd represents the voltage value written into the storage capacitor 23 via the data line 24. Once the scan signal G1 turns on the TFT 21, the voltage value Vd on the voltage line 24 is written into the storage capacitor 23, at which point the VCOM-AC is at a low level. When the scan signal G1 transitions from the high level to the low level, the voltage value Vd is pulled down under the influence of the parasitic capacitance, making it impossible to maintain the written data value. At this time, by transitioning the VCOM-AC from a low level to a high level, the level of the voltage value Vd will be pulled up, thereby mitigating the influence of the feed-through effect.
The pixel circuit 2 overcomes the feed-through effect by modulating the common voltage terminal 231 of the storage capacitor 23, i.e., by maintaining the voltage across the storage capacitor 23 at the originally written data voltage. Specifically, since the storage capacitor 23 has one terminal connected to the common voltage, the differential voltage across the storage capacitor 23 can be controlled by using an AC voltage to drive the common voltage terminal 231 of the storage capacitor 23, i.e., by switching the common voltage, to maintain the voltage value for driving the liquid crystal capacitor 22. Meanwhile, since the AC driving method modulates the voltage in response to data being written, the voltage swing of the data signal may be decreased accordingly. Because the power consumption is in direct proportion to the voltage swing, the decrease in the voltage swing of the data signal may result in the corresponding decrease in power consumption of the whole LCD.
However, because the AC voltage driving method needs to modulate the voltage according to the data signal, an additional driving circuit is needed to modulate the common voltage, thus adding to the cost. Therefore, it is still important to find a new LCD driving method which reduces the cost of manufacturing the driving circuits while still accomplishing the same functions.